As the demand for integrated circuits having ever-smaller device features continues to increase, the need for improved substrate inspection systems continues to grow. One aspect of inspection tool operation includes the control of polarization within the optical pathway of the inspection system implemented in order to analyze various aspects of defect, particle or surface features of a given sample, such as a semiconductor wafer. Traditional polarization control devices include electronically-controlled spatial light modulator devices. Typical spatial light modulators utilize liquid crystals, which currently are not suitable with UV light (λ<300 nm). In addition, previous spatial control of polarization within an optical pathway provides for discrete changes in polarization retardation. Such systems may result in a significant amount of stray light production due to scattering from pixel boundaries.
It is further noted that any polarization control mechanism must compete with other optical requirements, such as transmission, field of view, high numerical aperture, and control of aberrations. For example, an all-refractive element based optical system may result in a high number of such elements, which are all necessary to control different types of aberrations, which results in reduced system transmission, which, in turn, leads to higher cost. In addition, other optical designs, such as a parabolic mirror or simple Swartzschild objective, have intrinsic optical limitations, which, for example, may lead to various degrees of polarization aberration.
Another aspect of inspection tool operation includes the control of point spread function (PSF) at an imaging detector. Typical optical systems minimize PSF size by increasing numerical aperture (NA) and reducing aberrations in an effort to obtain a PSF as close to the diffraction limit as possible. However, in applications related to detecting small surface defects via light scattering techniques, the maximization of signal-to-noise ratio (SNR). One approach typically used to maximize SNR is to minimize unwanted noise originating from light scattered by a surface, while maintaining (or increasing) light from a defect of interest. Typically, this can accomplished via control of the transmitted polarization, and by mechanically limiting an open aperture. Transmission and suppression of polarizations of interest over a pupil plane may be controlled with single or multiple mechanical polarization elements in a pupil plane, with transmitted light having varying polarization across the pupil plane. However, such measures result in degrading PSF away from the diffraction limit. Therefore, it would be desirable to provide a system and method for curing defects such as those of the identified above.